UPPER ANISIAN TO LADINIAN AMMONOIDS FROM RASA (SAN SALVATORE DOLOMITE, WESTERN SOUTHERN ALPS, ITALY)

A rich ammonoid collection of 394 specimens from Rasa dolostone, a highly fossiliferous facies of the San Salvatore Dolomite, has been studied. The ammonoids were collected from 17 outcrops in the Rasa Valley (western Lombardy, Varese), a fossil locality known since the 19th century that was never studied in detail. Fitfeen genera, 21 species and eight taxa in open nomenclature are described. Flexoptychites, Ticinites, Aplococeras, Serpianites, Parakellnerites, Stoppaniceras, Nevadites, Chieseiceras, Hungarites, Eoprotrachyceras, Protrachyceras, Pompeckjites, and Proarcestes are already known, while Ceresioceltites and Rasaites are new. Ceresioceltites n. gen. includes small celtitids reported in literature from several Middle Triassic carbonate platforms of Southern Alps. Ceresioceltites fumagallii (Stabile in Stoppani, 1860, type), and C. paronai (Airaghi, 1912) are revised. The new and somewhat enigmatic genus Rasaites, that is assigned with doubt to the family Hungaritidae, is monotypic and based on R. rasaensis n. gen. n. sp. The last new species described is Flexoptychites gottardoi.The ammonoid faunas recognized in the 17 outcrops of Rasa dolostone succession document the Reitzi and Secedensis zones of the uppermost Anisian, as well as the Curioni and Gredleri zones of the lower Ladinian. The most fossiliferous interval is the Secedensis Zone. The occurrence of the Archelaus Zone within the stratigraphic succession cannot be confidently established, as a single specimen in open nomenclature may derive from this interval. The ammonoid succession of the Rasa dolostone is taxonomically and chronostratigraphically equivalent to the ammonoid succession of the Besano Formation described by Rieber in 1973 from Punkt 902/Mirigioli section (Monte San Giorgio, Switzerland), a locality that is about 15 km from Rasa. The specimens from Rasa, however, are preserved as body chambers and phragmocones filled by sediment, resulting in the preservation of suture lines and of the inner whorls. Therefore the Rasa ammonoids improve the systematics of the taxa from Punkt 902/Mirigioli, that was based on description of molds of body chamber, and cast of empty phragmocones. The ammonoid faunal similarity of Rasa dolostone with Besano Formation is surprising because the two successions were deposited in the same intraplatform basin (Besano basin, equivalent to the Grenzbitumenzone basin of Swiss authors), but in two very different settings: Rasa dolostone was deposited on the margin of the carbonate platform bounding the Besano basin, in shallow well oxygenated waters, while the Besano Formation was deposited on a deeper anoxic sea bottom.The fauna of the Secedensis Zone from the Platform margin of Rasa has been compared with the faunas of the same interval from the outer platform of Esino Limestone of Parina Valley (central Lombardy), and with the platform interior of Latemar (western Dolomites). Taking into account the very short duration of the Secedensis Zone, on the order of 350 kyr, this comparison demonstrate that ammonoids during the late Anisian were significant components of the carbonate platform ecosystems, with great adaptability to environments that are not usually considered as ideal for the group.